The Sign of Four (S04E01)

Years have passed since Miss Mary Morstan's father vanished, and each year she receives a single exquisite pearl from an unknown benefactor. When she finally receives an invitation to meet this enigmatic figure, she turns to Sherlock Holmes for assistance. As he investigates, Holmes uncovers a series of intriguing leads, including a deadly dart and a man with a prosthetic leg. Ultimately, he discovers an aging soldier intent on exacting revenge on those who wronged him and his comrades by seizing their valuable treasure from the East.